Generally speaking, a door is used to separate indoor from outdoor, and is always equipped with a lock by which we have to turn its knob to move a latch inwards so as to open the door. But, since turning the knob and opening the door won't always make a sound loud enough for persons indoors to hear it, somebody may hang a bell inside the door, which may be swung to ring while the door is opened so that the persons indoors can be warned that somebody is coming in. The object that this invention has aimed to attain is to obtain such an alarm at the same time that the knob is turned around.